从oci_num_rows php oracle返回零结果

When i execute this query the result of oci_num_rows always zero , can any one help ?!

  $sql="SELECT * FROM apps.xx_fa_by_loc_mob where site_name ='$location' AND  tag_number='$data1'";

$res= oci_parse($link, $sql);
oci_execute($res);

$var=oci_num_rows($res);
echo $var;

any help would be great !!

This might be the reason:

Note:

This function does not return number of rows selected! For SELECT statements this function will return the number of rows, that were fetched to the buffer with oci_fetch*() functions.